1. Field of the Invention
The present invention relates to a magnetic memory device and a method of manufacture of the same. More specifically, the present invention relates to the structure of wirings of a magnetic memory device.
2. Description of the Related Art
Magnetic memory devices that utilize magnetic properties are known. The magnetic memory devices store information utilizing the tunneling magnetoresistance (referred hereinafter to as TMR) effect.
One example of such magnetic memory devices is a so-called magnetic random access memory (MRAM). The MRAM, which is a generic name for solid memories that utilize a direction of a magnetization of a ferromagnetic material as an information recording carrier, can rewrite, hold and read recorded information whenever necessary.
FIG. 22A is a plan view schematically illustrating the structure of part of a typical magnetic memory device. FIG. 22B is a sectional view taken along line XXIIB—XXIIB of FIG. 22A. As shown in these figures, first and second write wirings 201 and 202 are arranged in a matrix form. A memory cell 203 is placed between the first and second write wirings 201 and 202 at each of intersections thereof.
As each of the memory cells 203 use is made of a MTJ element based on a magnetic tunnel junction (MTJ) (see, for example, ISSCC 2000 Digest Paper TA7.2). The MTJ element is comprised of a fixed layer, a tunnel barrier layer, and a recording layer, which are stacked in sequence.
In writing information into a selected memory cell, currents are caused to flow through the first and second write wirings associated with the selected cell. As a result, a magnetic field is produced at the intersection between the first and second write wirings, reversing the magnetization direction of the recording layer of the selected cell. Binary information is recorded depending on whether the magnetization direction of the recording layer is parallel or antiparallel with respect to that of the fixed layers. The intensity of a magnetic field (switching field) required to reverse the magnetization direction will be of the order of tens of Oe in a 0.1-μm rule by way of example.
The reading of recorded information is achieved by utilizing the magnetoresistance. The magnetoresistance is a phenomenon by which the electrical resistance of a memory cell changes according to the relative angle between the magnetization direction of the ferromagnetic body of the cell and current. A change in the resistance is read by causing current to flow in the memory cell.
As described above, in writing information into a selected memory cell 203 a current is caused to flow in each of the first and second write wirings 201 and 202. Memory cells in the neighborhood of the selected memory cell (hereinafter referred to as semi-selected memory cells) are affected by magnetic fields resulting from the currents in the wirings. As a consequence, the semi-selected memory cells may be written with erroneous information. This problem will become more serious as the integrated density of magnetic memory devices increases and hence the spacing between each memory cell decreases.
In order to prevent the semi-selected memory cells from being written with erroneous information, it may be effective to adjust the current value in each of the write wirings 201 and 202 associated with a selected memory cell. In order to generate the aforementioned switching field, however, it is required to cause a current of a predetermined magnitude in each of the write wirings. For this reason, a problem arises in that the tolerance for the current value at the time of writing is very small.